The Pentagon inspector general is investigating the Air Force's conclusions on a fatal F-22 crash, an unusual step for what are typically routine probes into aircraft accidents.

The IG sent a Jan. 25 letter to Air Force Secretary Michael Donley, saying the agency would investigate the Aircraft Investigation Board report into the November 2010 incident in Alaska, which killed Capt. Jeffrey Haney.

"Our assessment will also verify that AIB conclusions are supported by evidence of record consistent with standards of proof established by AFI 51-503," he wrote.

Defense Department IG spokeswoman Bridget Ann Serchak said in an email to media outlets that the DoD Inspector General "self-initiated" the investigation and that the office"does not comment on evaluations in progress."

Air Force Chief of Staff Gen. Norton Schwartz denied Thursday that the DoD investigation was anything but routine, adding that the probe "is not specific to this weapons system, but simply to this inquiry, and naturally we will support that completely."

He said an Air Force Scientific Advisory Board investigation into theF-22's onboard oxygen system is near completion and its findings should be available soon.

"Through that effort we have not identified a specific engineering fault but a number of ways that we can ensure that adequate oxygen in very high performance airplanes that operate over a very extensive altitude [range] protects the operators and maintains their physiological capacity to rock and roll," Schwartz said at a forum hosted by the Center for Strategic and International Studies, a D.C. think tank